3. Inclusion criteria: Children with MAS ranging from toddlers to young adults. Diagnosis of MAS will be made on a clinical basis. Blood testing is not helpful in this condition, as bone marrow progenitor cells with the Gαs mutation display a survival disadvantage. All patients are mixed chimeras, as this mutation is lethal if it occurs in germline cells. Patients who exhibit two or more of the following clinical findings fit the diagnosis of MAS: